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"It's pretty clear in adults that there is a greater increase in chronic respiratory disease, but it's very difficult to quantify that," he adds.
A common thread in the World Trade Center research has been a reliance on surveys. The city's World Trade Center Health Registry, for example, is basically a very large collection of surveys.
Some in the research community have raised questions about the validity of such data. Epidemiologists David Vlahov and Sandro Galea of the New York Academy of Medicine, for example, questioned the results of another study that used a self-administered survey to conclude that there were much higher rates of respiratory symptoms after 9/11 among the residents of lower Manhattan than among people on the Upper West Side. Vlahov and Galea caution that people in lower Manhattan would be more far likely to respond to the survey, since they have a greater interest in the results. Just over 2,300 people responded from lower Manhattan, and only 291 from the Upper West Side.
Because the survey was done some eight to 16 months after September 11, the accuracy of the memory of those surveyed is also questionable, the authors state.
"This study in some ways raises more questions than it answers," Vlahov and Galea write. "Inferences about individual exposures must be drawn with caution."
In April, using survey data from the registry, the city published a study of 8,418 survivors of the Twin Towers. That study found that half of those people reported one or more new respiratory symptoms after 9/11.
The authors of the study went on to list all of the problems with their conclusions. For one thing, they say, the total number of building survivors is unknown. For another, the data are based on the memories of people taking a survey two or three years after the fact. And the survey never established the boundaries of the dust cloud.
Finally, the people in the study were those who signed up for the registrypossibly biasing the sample. "This effect has been observed in other environmental studies in which persons who believed they were exposed tend to over-report health problems," the authors write.
In the spectrum of scientific knowledge, the strongest evidence of a link between the dust and respiratory illness is provided by the city's firefighters.
The FDNY had been collecting respiratory data on its members years before 9/11. As a result, doctors were able to compare the lung conditions of their patients before and after the attacks.
More than 3,000 firefighters have sought respiratory treatment since 9/11. Retirements based on lung problems have risen by four times the previous average. The observed drop in lung function after 9/11 was 12 times greater than the average annual decline in the five years before 9/11. More than 25 percent of firefighters showed symptoms of asthma or reactive airway dysfunction.
The most recent study of firefighters, published in March, concluded that 26 firefighters contracted a respiratory disease, sarcoidosis, in the five years after 9/11a rate higher than in the 15 years prior to 9/11. Half of those firefighters got the disease in the first year after the attack, and the remainder over the next four years.
"It's striking dataway above the norm even for their population," Carpenter says. For most of the other affected groups, however, data for the period prior to 9/11 generally do not exist, which makes it much more difficult to make judgments with the same kind of precision.
The New York Police Department, for example, is only now seeking funding to do a wide-ranging study of the thousands of police officers who worked at the site. That effort has been criticized by police unions for coming too lateand at a time when the NYPD is fighting disability claims.
In a study released last week, the city reported that 3.6 percent of 25,000 Ground Zero workers said that they had developed asthma after working at the sitea rate 12 times higher than that of the general population.
The survey also found that workers who were caught in the cloud or worked on the debris pile reported higher rates of asthma, and that those who wore dust masks reported asthma at a lower rate than those who did not. In other words, the higher your exposure, the greater your likelihood of reporting asthma.
The research evidence on severe respiratory disease, however, is limited. Other than the study of sarcoidosis in firefighters, there are just three other studies on severe respiratory disease, with each one examining a single specific case of three separate illnesses.
"It's hard to document and identify small elevations in rare events, so it takes a lot of work," says Lorna Thorpe, deputy commissioner of the city's health department. "Of the broader, more common ailmentscough, asthma, wheezingthey are very common in the general population, so it's not as easy to verify. The question of persistence of symptoms is of paramount importance."
The most persuasive study to date was released in September 2006 by the Mount Sinai WTC Medical Monitoring Program.
The study concluded that Ground Zero responders suffered exposure-related increases in respiratory- and pulmonary-test abnormalities that lingered up to 2.5 years after the attacks, with those who reached the site within the first two days of the attacks experiencing the worst symptoms.